1. Field of the Invention
The present invention relates to a unit and a method for driving a semiconductor device having a switching function. Specifically, the present invention relates to a unit and a method for driving a semiconductor device, capable of reducing a surge voltage while suppressing an increase in switching loss at the time of switching of the semiconductor device.
2. Related Art
Conventionally, in an electric vehicle, a synchronous electric motor driven by three-phase alternating current is typically used. Thus, the electric vehicle is provided with an inverter that converts the direct-current output from a battery (direct-current power source) into three-phase alternating current to drive the synchronous electric motor. The inverter provided in the electric vehicle will particularly be referred to as “electric vehicle inverter”.
Many electric vehicle inverters adopt PWM (pulse width modulation) control, and adopt an IGBT (insulated gate bipolar transistor) as a power semiconductor device for implementing the PWM control (see Patent documents 1 to 3).    Patent Document 1: JP-A-2007-306166    Patent Document 2: JP-A-2008-078816    Patent Document 3: US2010/0008113
The IGBT is a self-turn-off semiconductor device driven by a gate-emitter voltage Vge and capable of being turned on and off in response to an input signal to the gate.
Here, turn-off switching is switching of a state between a collector and an emitter of the IGBT from conduction state to shut-off state. Turn-on switching is switching of a state between the collector and the emitter of the IGBT from shut-off state to conduction state.
In the electric vehicle inverter, for such an IGBT, an FWD (free wheeling diode) is used so as to be paired therewith. That is, the FWD is a free wheeling diode for the IGBT, and is connected in parallel to the IGBT in a direction opposite to the input and output direction of the IGBT.
Moreover, in the electric vehicle inverter, a circuit that drives the IGBT (hereinafter, referred to as “semiconductor device driving circuit”) is provided. That is, the semiconductor device driving circuit controls the turning on and off of the IGBT by changing the value of the gate-emitter voltage Vge of the IGBT.
However, a surge voltage occurs during the transition period at the time of switching such as the turning on or off of the IGBT. Hereinafter, brief description of the surge voltage will be given.
A floating inductance is present in a circuit (bus) to which the IGBT is connected. Such a floating inductance becomes an inertial force on the current, and acts in such a way as to hinder the current from changing. Therefore, if the current behaves in a way as to abruptly decrease, in the floating inductance, an electromotive force occurs in a direction that hinders the current from decreasing. That is, in the electric vehicle inverter, an electromotive force occurs in a direction in which it is added to the power supply voltage of the battery in series. The voltage based on the electromotive force occurring in this way is called “surge voltage”.
In the electric vehicle inverter, with two series-connected IGBTs as one unit, for three phases of load of the synchronous electric motor, a plurality of units, for example three units, are used in a state of being connected in parallel. Within one unit, when one IGBT is turned on, the other IGBT is turned off. Therefore, during the transition period at the time of switching within one unit, since the collector current of one of the IGBTs abruptly decreases, a high surge voltage occurs and is superimposed on the power supply voltage, and this is applied between the collector and emitter of the IGBT.
For this reason, it is necessary for the IGBT to have an element withstand voltage capable of tolerating such a surge voltage. Therefore, naturally, the higher the surge voltage is, the higher the required element withstand voltage is, so that the size of the IGBT increases. In the case of industrial inverters used in plants and the like, since there is sufficient installation space in plants, a large-size IGBT can be adopted. However, in the case of the electric vehicle inverter, it is difficult to ensure such installation space in electric vehicles, so that it is extremely difficult to adopt a large-size IGBT.
Therefore, it is required that the IGBT provided in the electric vehicle inverter be downsized. To downsize the IGBT, conversely, the element withstand voltage is held down. To do this, the surge voltage is reduced.
As described above, since the surge voltage occurs due to abrupt reduction in current, the surge voltage can be reduced by making sluggish the degree of change of the current reduction. That is, when the rise and fall times of the current and voltage at the time of switching of the IGBT will hereinafter be called “switching speed”, the surge voltage can be reduced by reducing the switching speed.
However, if the switching speed is reduced to reduce the surge voltage, the loss of the IGBT and FWD during the transition period at the time of switching (hereinafter, referred to as “switching loss”) is increased in turn.
On the other hand, if the switching speed is increased to reduce the switching loss, the surge voltage is increased as described above.
As described above, there is a relationship of trade-off (trade-off requirement) between the surge voltage and the switching loss. Hereinafter, the characteristic of the surge voltage and switching loss having such a relationship therebetween will be referred to as “trade-off characteristic of the surge voltage and switching loss”.
Therefore, it is demanded of the electric vehicle inverter that the trade-off characteristic of the surge voltage and switching loss be improved, in other words, the surge voltage be reduced while the increase in switching loss is suppressed at the time of switching of the IGBT.
To satisfy this demand, some methods are disclosed in Patent Documents 1 to 3. However, it is difficult to say that these conventional methods sufficiently satisfy the demand. Therefore, the situation is such that a new method capable of sufficiently satisfying the demand is required.
While the electric vehicle inverter has been described as an example, downsizing is required not only of the electric vehicle inverter but also of various devices adopting semiconductor devices having a switching function. Therefore, the situation is such that the new method capable of sufficiently satisfying the demand is also required to be not only applicable to the IGBT of the electric vehicle inverter but also widely applicable to semiconductor devices having a switching function.